Sol-gel Silicas Research Articles

The influence of synthesis conditions, oligosaccharide additive and functional silane on the structure and composition of sol–gel silicas

The influence of synthesis conditions, oligosaccharide additive and functional silane on the structure and composition of sol–gel silicas

Sol-gel hybrid silicas as an useful tool to mercury removal

Due to high potential toxicity of mercury species and their characteristics of bioaccumulation and biomagnification, these pollutants have become a huge problem for environment, being able to spread along all trophic levels, including humans. This work describes the influence of hybrid silicas textural characteristics on mercury uptake capacity, revealing the possibility to use different sol-gel routes as a form to produce materials to interact with mercury species. The hybrid silicas prepared by basic catalyzed route showed more efficiency to remove inorganic mercury from aqueous media (1.5–1.9 μg. mg−1 of adsorbed mercury) if compared to those obtained by acid-catalyzed or two-steps routes (zero μg. mg−1 of adsorbed mercury). In addition, the hybrid silicas loaded with amino or sulfur-groups showed the high capacity to adsorb mercury, removing 82–99% from a 10 mg.L−1 Hg(II) solution with only 50 mg of the sorbents.

Accelerated laboratory weathering of polypropylene composites filled with synthetic silicon-based compounds

Non-functionalized and n-alkyl functionalized polyhedral oligomeric silsesquioxanes (POSS), siloxane-silsesquioxane resins and the sol-gel silicas were for the first time examined as possible UV-stabilizers and/or antioxidants in polypropylene (PP). The obtained composites were subjected to the accelerated laboratory weathering tests. The influence of the weathering conditions on the structure and properties of the PP materials was evaluated on the basis of the results from Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and differential scanning calorimetry (DSC) studies. It was found that weathering resistance of the PP materials was strongly depended on the kind of the silicon-based filler used. Functionalized silsesquioxanes and siloxane-silsesquioxane resins improved weathering resistance of the PP materials, most probably because of its uniform dispersion in polymer, and thus, they could be considered as antioxidant additives. The changes in structural characteristics influenced thermal properties of the PP composites. It was revealed that the addition of the n-alkyl functionalized POSS and siloxane-silsesquioxane resins caused less significant changes in the melting and crystallization behaviour of PP materials.

Evaluation of the pozzolanicity of nanostructured sol-gel silica and silica fume by electrical conductivity measurement

The present paper evaluates the pozzolanic activity of two nanostructured silicas with high specific surface area, synthetized by sol-gel technique, in comparison with silica fume. The materials were characterized by X -ray diffraction, scanning electron microscopy and nitrogen sorption. Electrical conductivity tests were performed in calcium hydroxide saturated solutions. Results revealed that both nanostructured sol-gel silicas have great pozzolanic reactivity, much higher than silica fume, especially in the early stage of the reaction. Hence, the pozzolanic reactivity of silica is affected by its specific surface area. Moreover, silica amount, test temperature and average pore size also seems to alter the silica reactivity. The reaction rate of silicas with small average pore size is reduced along the pozzolanic reaction.

ChemInform Abstract: Catalysis via Sol‐Gel Acid Silicas: An Important Chemical Technology for 2nd Generation Biorefineries

AbstractReview: [57 refs.

Combining mercury thermoporometry with integrated gas sorption and mercury porosimetry to improve accuracy of pore-size distributions for disordered solids

The typical approach to analysing raw data, from common pore characterization methods such as gas sorption and mercury porosimetry, to obtain pore size distributions for disordered porous solids generally makes several critical assumptions that impact the accuracy of the void space descriptors thereby obtained. These assumptions can lead to errors in pore size of as much as 500%. In this work, we eliminated these assumptions by employing novel experiments involving fully integrated gas sorption, mercury porosimetry and mercury thermoporometry techniques. The entrapment of mercury following porosimetry allowed the isolation (for study) of a particular subset of pores within a much larger interconnected network. Hence, a degree of specificity of findings to particular pores, more commonly associated with use of templated, model porous solids, can also be achieved for disordered materials. Gas sorption experiments were conducted in series, both before and after mercury porosimetry, on the same sample, and the mercury entrapped following porosimetry was used as the probe fluid for theromporometry. Hence, even if one technique, on its own, is indirect, requiring unsubstantiated assumptions, the fully integrated combination of techniques described here permits the validation of assumptions used in one technique by another. Using controlled-pore glasses as model materials, mercury porosimetry scanning curves were used to establish the correct correspondence between the appropriate Gibbs–Thomson parameter, and the nature of the meniscus geometry in melting, for thermoporometry measurements on entrapped mercury. Mercury thermoporometry has been used to validate the pore sizes, for a series of sol–gel silica materials, obtained from mercury porosimetry data using the independently-calibrated Kloubek correlations. The pore sizes obtained for sol–gel silicas from porosimetry and thermoporometry have been shown to differ substantially from those obtained via gas sorption and NLDFT analysis. DRIFTS data for the samples studied has suggested that the cause of this discrepancy may arise from significant differences in the surface chemistries between the samples studied here and that used to calibrate the NLDFT potentials.

Synthesis of novel Schiff base doped sol–gel silicas

Sulfonamide Schiff bases were doped uniformly in silica sol–gels prepared from liquid precursors by a fast and easy way at room temperature and processed to form xerogels. Schiff bases are efficient chelating agents, bioactive and catalytically active compounds. The structures of the newly synthesized Schiff base doped xerogels were elucidated by their physical (morphology, surface area, porosity), spectral (FTIR) and analytical (CHNSO/Si) data. The powder X-ray diffraction studies were carried out to confirm the formation of single phase. Characterization confirmed that Schiff base molecules are entrapped inside the pores as well as physically bound onto the silica surface. All Schiff base doped xerogels are stable mesoporous materials showing hydrophilic properties. Loadings of Schiff bases from 0.10 to 0.23 g/g of xerogel were obtained resulting amorphous materials. The doping of Schiff bases with xerogel caused change in surface area, pore volume and pore diameter of xerogel without damaging the main framework of siliceous skeleton. Morphology and colour of xerogel was also changed after doping. The entrapment of Schiff bases in xerogel caused increase in their decomposition temperatures. The final Schiff base doped xerogels show remarkable thermal stability.

Clathrate formation and dissociation in vapor/water/ice/hydrate systems in SBA-15, sol–gel and CPG porous media, as probed by NMR relaxation, novel protocol NMR cryoporometry, neutron scattering and ab initio quantum–mechanical molecular dynamics simulation

The Gibbs-Thomson effect modifies the pressure and temperature at which clathrates occur, hence altering the depth at which they occur in the seabed. Nuclear magnetic resonance (NMR) measurements as a function of temperature are being conducted for water/ice/hydrate systems in a range of pore geometries, including templated SBA-15 silicas, controlled pore glasses and sol-gel silicas. Rotator-phase plastic ice is shown to be present in confined geometry, and bulk tetrahydrofuran hydrate is also shown to probably have a rotator phase. A novel NMR cryoporometry protocol, which probes both melting and freezing events while avoiding the usual problem of supercooling for the freezing event, has been developed. This enables a detailed probing of the system for a given pore size and geometry and the exploration of differences between hydrate formation and dissociation processes inside pores. These process differences have an important effect on the environment, as they impact on the ability of a marine hydrate system to re-form once warmed above a critical temperature. Ab initio quantum-mechanical molecular dynamics calculations are also being employed to probe the dynamics of liquids in pores at nanometric dimensions.

Functionalized sol–gel material for extraction of mercury (II)

1,5 Diphenylcarbazide (DPC) functionalized sol–gel silicas (DPCSG) were prepared and investigated for the solid phase extraction of Hg(II) from aqueous solution by a batch equilibrium technique. The metal sorption properties of DPCSG were examined to evaluate their performance toward metal extraction under different controlling factors, such as pH of solutions, equilibration shaking times, effect of foreign ions and complexing agents. The equilibration was fast and was achieved within 30 min. Maximum extraction capacity of Hg(II) ions onto the DPCSG from non-competitive solutions was 5.6 mg/g (0.028 mmol/g) at 28 ± 2 °C and the enhancement factor was found to be 560 at optimized pH 6 when compared with control and commercial silica gel. The adsorption data obtained as a function of mercury concentration were fitted to a Langmuir adsorption model. The stripping and regeneration of the sorbent were reported and it was further observed that extraction and regeneration were indicated by the dark violet colouration and decoloration of the extractant, respectively. It is concluded from the extraction data that DPCSG is a promising material for the extraction of mercury.

Probing surface interactions by combining NMR cryoporometry and NMR relaxometry

To further expand on the understanding of surface interactions at the liquid/solid interface on pore walls, the nuclear magnetic resonance (NMR) techniques of cryoporometry and relaxometry have been combined. The combination of these techniques allows variations in NMR relaxation parameters from pore surface to volume ratio changes and from surface interaction changes to be distinguished. By studying a range of sol–gel silicas from two different sources, it was noted that the relaxation time measurements were not consistent with the pore diameters determined by cryoporometry and N2 gas adsorption. Instead distinctly different relaxivity constants were determined for each absorbate in each of the two brands of silica. It was clear that the relaxation times were modified by more than just the pore geometry. Independent experiments on the two brands of silica suggested that the relaxometry results were heavily influenced by the concentration of paramagnetic relaxation centres in the silica gels. The strength of surface interaction, and hence surface affinity, was seen to depend on the liquid in the pores. Using this difference in surface affinities, binary mixtures of alkanes placed in sol–gel silicas were separated via preferential absorption and their components identified using cryoporometry, whereas the components could not be distinguished in the bulk liquid.

Structural and dynamic studies of water in mesoporous silicas using neutron scattering andnuclear magnetic resonance

Experimental techniques for studying the behaviour of water in confined geometry usingneutron scattering and NMR methods are reviewed. A brief survey is given of earlier workon sol–gel silicas and these findings are updated by reference to current work on MCM- andSBA-type silicas. Particular attention is focused on the phase transitions and the relationbetween supercooled water and ice phases in the confined geometry of the mesopores. Thecharacteristics are found to behave in a systematic manner although the nucleationphenomenon for partially filled pores reveals some unexpected complexity for the SBAsilicas.

Characterisation of porous solids using small-angle scattering and NMR cryoporometry

The characteristics of several porous systems have been studied by the use of small-angle neutron scattering (SANS) and nuclear magnetic resonance (NMR) techniques. The measurements reveal different characteristics for sol–gel silicas, activated carbons and ordered mesoporous silicas of the MCM and SBA type. Good agreement is obtained between gas adsorption measurements and the NMR and SANS results for pore sizes above 10 nm. Recent measurements of the water/ice phase transformation in SBA silicas by neutron diffraction are also presented and indicate a complex relationship that will require more detailed treatment in terms of the possible effects of microporosity in the silica substrate. The complementarity of the different methods is emphasised and there is brief discussion of issues related to possible future developments.

Binary liquid mixtures in porous solids.

The technique of nuclear magnetic resonance cryoporometry has been used to study the behavior of binary liquid mixtures of water and decane in porous sol-gel silicas. It was observed that the water preferentially adsorbed onto the silica surface and so was able to displace the decane from the pores.

Determination of the cause of mercury entrapment during porosimetry experiments on sol–gel silica catalyst supports

In previous work, a modelling methodology was developed to determine statistical information, similar to that which can be obtained from 1 H MRI, on the spatial distribution of different pore sizes within porous media using mercury porosimetry. The new methodology has the advantage over MRI that it is suitable for application to chemically heterogeneous materials of interest in catalysis, such as coked catalysts and supported metals, that are not amenable to quantitative studies using conventional 1 H MRI. However, the new methodology relied upon the theory that the entrapment of mercury within many porous solids, such as sol–gel silicas, occurs because the spatial distribution of pore sizes within the material is not random, but is, in fact, highly correlated. Mercury entrapment is thought to occur in isolated, macroscopic (>10 μm in size) domains, containing similarly sized larger pores, that are completely surrounded by continuous networks of smaller pores. In this work mercury porosimetry experiments on silicas, consisting of a primary intrusion and retraction cycle, followed by re-injection of mercury and secondary retraction, have shown that the entrapment of mercury does not arise due to the alternative possibility of irreversible pore structural collapse. Light microscopy studies of transparent samples following mercury porosimetry experiments have confirmed that the spatial distribution of entrapped mercury is highly heterogeneous and occurs in particular macroscopic regions within the sample. These findings support the underlying theory of mercury entrapment used in previous simulations of the mercury porosimetry experiment.

An evaluation of NMR cryoporometry, density measurement and neutron scattering methods of pore characterisation

Sol-gel silicas with nominal pore diameters ranging from 25A to 500A were studied by NMR cryoporometry, and by neutron diffraction and small angle scattering from dry silicas over the Q range 8. 10(-4)A(-1) < or = Q < or = 17A(-1). Density and imbibation experiments were also performed. Geometric models of porous systems were constructed and were studied by both analytic techniques and Monte-Carlo integration. These models, combined with the information from the above measurements, enabled the calculation of the fully density corrected solid-solid density correlation functions G(r) for the sol-gel silicas, deduction of the (voidless) silica matrix density, measurement of the silica fraction in the grain and of the packing fraction of the silica grains and an estimation of the water equivalent residual hydrogen on the dried silica surface. In addition, the pore diameter D, pore diameter to lattice spacing ratio D/a, and pore and lattice variance sigma could also be measured. While the NMR cryoporometry pore diameter measurements for the sol-gel silicas show excellent co-linearity with the nominal pore diameters as measured by gas adsorption, and the calculated pore diameters from the measured neutron scattering show surprisingly good agreement with these measurements at large pore diameters, there is a divergence between the calibrations for pore diameters below about 100A.

Organized Functionalization of Mesoporous Silica Supports Using Prefabricated Metal-Polymer Modules

Porous silica frameworks containing well-dispersed metal particles are particularly important in catalysis. These authors report a method for direct inclusion of metal nanoparticles into a host framework, thus circumventing the need for post-synthesis functionalization of the support. The method involves a new molecular hybrid templating technique that exploits the ability of the functionalized polymer microgel to function as both an exotemplate (controlling the growth of the metal colloids) and as an endotemplate (during the casting of the mesoporous sol-gel silicas), thus allowing the 3D arrangement of metal particles to be controlled.

Organized Functionalization of Mesoporous Silica Supports Using Prefabricated Metal–Polymer Modules

Porous silica frameworks containing well-dispersed metal particles are particularly important in catalysis. These authors report a method for direct inclusion of metal nanoparticles into a host framework, thus circumventing the need for post-synthesis functionalization of the support. The method involves a new molecular hybrid templating technique that exploits the ability of the functionalized polymer microgel to function as both an exotemplate (controlling the growth of the metal colloids) and as an endotemplate (during the casting of the mesoporous sol-gel silicas), thus allowing the 3D arrangement of metal particles to be controlled.

Acid and Base Catalysts in the Hybrid Silica Sol–Gel Process

Sol–gel silicas were synthesized from the hydrolysis of tetraethoxysilane andN-β-aminoethyl-γ-aminopropyltrimethoxysilane in the presence of water and ethanol by using ammonium hydroxide or hydrochloric acid as base or acid catalysts, respectively. The influence of the type of the catalyst employed was studied by comparing the surface areas and pore sizes and volumes for both sol–gel silicas. Nitrogen elemental analysis and13C and29Si NMR spectra showed that both silicas present similar chemical compositions and structures. The electron-microscope results demonstrated that the particles are uniformly spherical and well shaped in both cases. The sol–gel silica obtained via basic catalysis presented relatively small spherical particles in comparison to those synthesized by acidic catalysis. The results obtained suggest that acidic catalysis is a better procedure for the synthesis of these organofunctionalized hybrid sol–gel silicas.

Energetics and structure of sol-gel silicas

High temperature calorimetry was used to measure the metastability, relative to fused silica glass, of an acid and a base catalyzed silica gel. This excess enthalpy can be separated into two parts, a ‘fast’ release enthalpy which occurs on heating the gel to 700–800 °C and a ‘slow’ release enthalpy detectable only when the Si O Si bridges are broken on dissolving the sample in molten lead borate. When the effects of H 2O, OH, and OC 2H 5 are subtracted by appropriate thermochemical cycles, the silica framework of the acid catalyzed gel is found to be metastable by 7.3 kJ/mol and that of the base catalyzed gel by 66.8 kJ/mol, with ‘fast’ release enthalpy accounting for most of the effect. The extremely large release of heat in the base catalyzed gel is proposed to be due mainly to the relaxation and rearrangement of metastable siloxane rings created from rapid condensation of monomers (due to high solubility of silica in basic solution) during drying. The much smaller metastability of the acid catalyzed gel may be associated with pore collapse as volatiles are removed. This proposal is consistent with chemical analyses, TGA/DSC and photoacoustic infrared spectroscopy of the materials.

ESR studies of irradiated 17O-enriched sol-gel silicas : Organic impurity effects and the structure of the nonbridging-oxygen hole center

ESR studies of irradiated 17O-enriched sol-gel silicas : Organic impurity effects and the structure of the nonbridging-oxygen hole center